I don't understand why people don't get the difference.. At the point when Will was saying Kieran was a traitor, who was doing so on exactly the same basis as everyone else, i.e. saying that that was his guess because X, Y and Z. His views on it held no more weight than those of the other faithful because he wasn't claiming to have any special knowledge. And throughout the game he has of course had to nominate people, just as all the participants have to.
On the other hand, Kieran was plainly angry, and nominated Will and said it was his parting shot at a time when he had nothing to gain within the game by doing so - and a minute later said he (Kieran) was a traitor. He could hardly have done more to shop Will if he had held up a neon sign saying "Yes, I know for a fact he is a traitor on the most authoritative basis possible, because I am one". Will was saying that, like the three faithful, he was guessing Kieran was a traitor; Kieran was saying he 100% knew he was because he worked with him as a fellow-traitor.
At the point when someone has been banished and admitted he is a traitor, why would he want revenge on any of the faithful? That's what the game is all about, after all. Plus, if there was another traitor who had denounced and nominated him, why would he let her carry on to win purely for the sake of getting revenge on a faithful?
If traitors were allowed to send out big obvious hints when they get banished, then as soon as the first traitor went she could have identified they other two and they would have been picked off the next two evenings. By then, presumably, they would have had to recruit, but they could denounce their recruits when they go, and so on. The entire game would collapse in chaos within days.